Field of the Invention
The invention relates generally to the field of wind turbines, and more specifically to improved blade designs that increase power, efficiency, and reliability. The blades may be utilized in a variety of applications, including wind turbines and turbine engines.
Description of Related Art
Wind turbines are an important component of renewable sources of energy generation. Wind turbine technology has been applied to large-scale power generation applications. Of the many challenges that exist in harnessing wind energy, one is maximizing wind turbine performance while ensuring reliability and broad applicability. Non-limiting examples of improved wind turbine performance parameters, which lead to minimized cost of energy, include maximized aerodynamic efficiency, maximized energy output, minimized wind turbine system loads, minimized noise, and combinations thereof.
The blade profiles are the most important factor in determining a wind turbine's performance and efficiency. The location which a turbine will be installed is also significant. In addition to wind potential, the wind structure is crucial too. Some factors, such as roughness of the surface, field, altitude, air pressure, temperature, density of the air, gust of wind, the direction and speed of the wind, are of great importance for turbines to work properly.
In practical designs, inefficiencies in the design and frictional losses will reduce the power available from the wind still further. Converting this wind power into electrical power also incurs losses of up to 10% in the drive train and the generator and another 10% in the inverter and cabling. Furthermore, when the wind speed exceeds the rated wind speed, control systems limit the energy conversion in order to protect the electric generator so that ultimately, the wind turbine will convert only about 30% to 35% of the available wind energy into electrical energy.
Such a conversion efficiency is in line with aerodynamic theory. German aerodynamicist Albert Betz showed that a maximum of only 59.3% of the theoretical power can be extracted from the wind, no matter how good the wind turbine is, otherwise the wind would stop when it hits the blades. He demonstrated mathematically that the optimum extraction occurs when the rotor reduces the wind speed by one third. However, the disclosed embodiments represent a new technique for exceeding this theoretical limit, and thus represent a major step forward for the field.
One objective in wind turbine design is to maximize aerodynamic efficiency, or power extracted from the wind. The number of blades in the turbine rotor and their rotational speed must be optimized to extract the maximum energy from the available wind. While using rotors with multiple blades should capture more wind energy, there is a practical limit to the number of blades which can be used because each blade of a spinning rotor leaves turbulence in its wake, which reduces the amount of energy the following blade can extract from the wind. This same turbulence effect also limits the possible rotor speeds because a high speed rotor does not provide enough time for the air flow to settle after the passage of a blade before the next blade comes along. There is also a lower limit to both the number of blades and the rotor speed. With too few rotor blades, or a slow turning rotor, most of the wind will pass undisturbed through the gap between the blades reducing the potential for capturing the wind energy. The fewer the number of blades, the faster the wind turbine rotor needs to turn to extract maximum power from the wind. This can be a disadvantage as it limits the areas in which a turbine can be placed to those with relatively high average wind speed.
It is helpful to view a blade profile as an airfoil cross section from root to tip. The forces generated may be expressed as the sum of two tangential forces. The force which drives the wind turbine, lift force, is generated when wind flows over the airfoil. Lift force is perpendicular to apparent velocity of the wind. Generally lift force increases with angle of attack, that is, the relative orientation of the airfoil surface to the motion of the wind. The second force component, drag force, also increases with angle of attack. While the lift force supports blade rotation, the drag force opposes it. Thus, wind turbine performance is maximized when the lift to drag ratio is maximized. The angle at which this occurs is called the optimum angle of attack. Airfoil cross sections are aligned in a way to operate at this optimum angle of attack.
Apparent wind velocity is the wind experienced by an observer in motion, in this case the blade profile surface, and is the relative velocity of the wind in relation to the observer. Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air. Even though wind velocity is uniform along the length of the blade, blade velocity increases linearly as we move towards the tip. Thus, angle and magnitude of the apparent wind velocity varies along the length of the blade. Apparent velocity becomes more aligned with the chord direction towards the tip of the blade. Because of this variation in apparent velocity, it may be preferable to twist the blade profile such that every portion of the airfoil cross section operates at its optimum angle of attack while the blade is rotating about the rotor in the wind.
With the increasing popularity of wind turbines, there is a need for improved blade profile designs that increased the efficiency and durability of the overall wind turbine. It would also be advantageous if the profile was operable at lower minimum velocities, as that would expand the potential for wind installations to areas that where winds were previously considered too slow to efficiently harness.
There are a wide variety of wind turbine blade designs. In addition to specific designs by individuals or corporations, there are standardized blade profiles defined by NACA and Gottingen, for example the NACA 0015 and Gottingen 622 airfoils. There are numerous variations on single profile designs, however the art does not teach two blade profiles physically coupled together. For example, WIPO Patent Application WO/2012/007934, filed Jul. 7, 2011, “relates to a wind turbine having inner and outer blades for directing wind flow towards each other.” The inner and outer blades are separate from each other and rotate in opposite directions.
U.S. Pat. No. 5,161,952, entitled “Dual-plane blade construction for horizontal axis wind turbine rotors,” to Eggers, Jr., describes a wind turbine with blades that are joined together at both the rotor and tip.
U.S. Patent Application Pub. No. 2015/0050159 to Caldeira, et al., describes turbine engines with a blade design in which the “turbine blade further includes a shell disposed around the core element, and the volume between the core element and the shell forms a void.”
WIPO Patent Application WO/2012/007934, by Mordechai Cohen, states that “the present invention relates to a wind turbine having inner and outer blades for directing wind flow towards each other.” In other words, Cohen describes an arrangement where two independent sets of blades are set in two shafts and are allowed to rotate freely from each other.
U.S. Pat. No. 7,387,491 to Saddoughi, et al., describes “active flow modification in wind turbines for reducing loads, reducing aerodynamic losses, improving energy capture, reducing noise, and combinations thereof.”
U.S. Pat. No. 8,246,311, entitled “Wind turbine rotor blade with variably actuatable porous window” and filed Dec. 7, 2010 by David Samuel Pesetsky, describes a porous window that may be slid along the chord of the blade.
U.S. Pat. No. 7,435,057, entitled“Blade for wind turbine” and filed Jul. 13, 2005 by Jorge Panera, discloses a hollow blade with an active fan mounted inside the blade to actively create a high velocity air current.
U.S. Pat. No. 8,747,070, entitled“Spinning horizontal axis wind turbine,” by Greg E. Blonder, discloses “A spinning horizontal axis wind turbine is disclosed. The blades of the wind turbine are configured to allow the blades to simultaneously rotate in a vertical axis and a horizontal axis when acted upon by an external force such as a wind current. The tip of each blade travels along a helical ‘figure 8’ pattern as the blade rotates through a complete cycle, moving from nearly vertical to nearly horizontal in a complete cycle.”
Despite the improvements shown in the above described designs, there is still a need for improved wind turbine profiles and profile designs that approach or exceed the Betz theoretical efficiency limit.